The present invention relates to the field of power distribution systems managing electrical power distributed between connected power sources and energy storage element over a regulated bus. More particularly, the present invention relates to distributed power sources and stores in a power distribution system.
Microsatellites and Nanosatellites in low earth orbits require the collection of sufficient power for onboard instruments and are low weight and low volume satellites. Because the overall surface area of a microsatellite or nanosatellite is small, body-mounted solar cells are incapable of providing enough power. Deployment of traditional, rigid, solar arrays necessitates larger satellite volumes and weights, and also requires extra apparatus needed for pointing toward the sun to collect solar energy. Nanosats are small orbital satellite becoming increasingly used for space communications because of the decreased deployment costs and lightweight. Nanosats inherently have a limited amount of available power because of a limited amount of space for mounting solar cells. Additionally, because of the small size of the nanosatellite, there is a weight penalty when deploying rigid solar panels with tracking and pointing mechanisms. Satellites have long used means for stowing and deploying a large area of solar cells with minimum weight and volume. This has been accomplished by utilizing solar cells on deployable flat panels that require pointing and that can be sized for various desired power levels. Deployable flat panels disadvantageously require pointing and tracking means as well as rigid deployable flat panels.
In traditional space power systems, individual DC power source devices such as solar cells, and DC energy storage devices such as batteries, have been connected in a series to develop sufficient voltage levels to be useful for supplying power to loads on the satellites. Series connections have been used in both regulated and unregulated buses within a centralized power management and distribution system. Traditionally, individual solar cells have been connected in a series to develop sufficient voltage and are then delivered to the power distribution system. A regulated bus is one where a precise voltage level is maintained and supplied to the loads. To maintain energy balance, the voltage regulator must throttle the amount of current supplied to the bus as required by the loads at each instant of time. Typically, the regulation of the amount of current that a series connection of power sources supply to the bus has been accomplished by the use of shunt dissipators. These dissipators sense the bus voltage and determine whether the voltage status level is low, indicating that the amount of current being supplied is low, or the voltage status level is high, indicating that the amount of current being supplied is high. Regulated adjustments are then made in the amount of current being supplied to maintain a constant voltage on the regulated bus.
Many power sources and energy storage devices operate efficiently and can be managed better for longer life if controlled at the individual device level. A solar panel comprises a plurality of parallel connected strings each of which comprises a plurality of series connected solar cells. For solar cells connected in a series, the weakest cell in the series provides the least amount of current. The weakest series connected solar cell will limit the power output of all of the other solar cells in that series. Hence, the weakest solar cell in the series will limit overall efficiency of that string. In addition, if the current mismatch between the weakest cell and all the other solar cells becomes too great, then the weakest cell will be driven into reverse bias, which could cause damage to the cell and eventual failure of the entire string. To avoid this failure, bypass diodes have been used to shunt current around the affected cell effectively disconnecting it from the string.
Similarly, energy storage devices must be current-matched so they will all charge and discharge at the same rate. The charge cycle is more critical in that overcharging at a high rate can cause damage to the individual storage devices. Each series of storage devices may include a network of bypass electronics as standard procedures on spacecraft to control the charging of individual energy storage devices. Energy storage devices are also typically connected in series. In the event of a failure of one of the storage devices, the entire string of series connected storage devices will fail. Bypass diodes are used to remove from a string one or more of the storage devices, then the string will not produce the desired voltage level, thereby creating a mismatch between operational and failed strings. The voltage level mismatch results in unequal load sharing between the parallel strings of storage devices. A string with a bypassed failed storage device will provide a lesser voltage level than the fully operational strings of storage devices, and the string with the failed storage device will contribute less to the supply of power delivered by the remaining operational strings, thereby reducing overall storage capacity of the storage devices. Further still, satellite power distribution systems typically operate using a single regulator for a string of connected devices and a failure of any one of the devices can cause a catastrophic system failure without the addition of redundant regulators with the attendant addition in complexity and weight. These and other disadvantages will be solved or reduced using the present invention.
An object of the invention is to collect, store and or distribute power within a power distribution system.
Another object of the invention is to efficiently collect solar power using solar cells deployed on a satellite.
Yet another object of the invention is to conform solar cells to a curved surface with each cell receiving differing amounts of solar illumination providing respective differing amounts of unequal power efficiently managed by a power management system.
Still another object of the invention is to provide a power management system having a plurality of DC power sources and DC energy stores connected to a common regulated bus through respective regulators.
A further object of the invention is to provide a power management system having a plurality of indivisible DC power sources and or indivisible DC energy stores connected to a common regulated bus through respective regulators for operationally isolating the sources and stores from each other.
Yet a further object of the invention is to provide a power management system having a plurality of indivisible DC power sources and or indivisible DC energy stores connected to a common regulated bus through respective regulators for operationally isolating the sources and stores from each other enabling graceful degradation of power distribution in the event that any one or more of the sources or stores fail.
The present invention is directed towards a power distribution system particularly useful for satellites, including microsatellites and nanosatellites. In one form, a deployable power sphere having a curved surface is preferably used to support attached solar cells that may be, for example, disposed in a grid along longitudes and latitudes corresponding and conforming to the shape of a sphere and are used to collect solar energy for a satellite. The solar cells could also be deployed in other grids, such as a hexagonal grid, or in a random arrangement, but conforming to the curved exterior of the power sphere. The power sphere itself need not be a perfect sphere, and may assume any volumetric shape, so long as the solar cells conformed to the exterior curved surface. The solar energy illuminates the conforming solar cells with uneven radiation intensity, and the power distribution system serves to collect this uneven intensity radiation with a high degree of efficiency. The power sphere offers a solution of providing adequate electrical power from unevenly illuminated solar cells to loads in a satellite. The power distribution system and the spherically disposed solar cells enable arbitrary orientation t the sun while efficiently collecting solar energy.
The power distribution system is configured with the individual indivisible DC power sources and DC energy stores connected in parallel to a regulated power bus. The power distribution system is preferably a five-watt power system using solar cells as DC power sources and lithium batteries as DC energy stores. The power sources and energy stores are indivisible DC sources and stores. The parallel connection is accomplished through respective microelectronic DC-DC regulators for each power source or energy store. This parallel connection of indivisible sources and stores eliminates the need for a series connections of these sources and stores in strings to thereby isolate the indivisible sources and stores from each other while efficiently producing a sufficient voltage level to supply the regulated bus supplying power to the connected load. The use of integrated circuit technology allows the package each of these regulators on a single chip. This integration makes it possible to deploy a large number of individual regulators, each with respective control circuitry, for isolating from each other the power sources and energy stores. In the event that one or more of the indivisible DC power source or energy stores fail, the power distribution system energy storage and power collection capabilities gracefully degrade while maintaining desired regulated voltage levels on the regulated bus.
The preferred solar power sphere is a spherically shaped solar array that will collect incident sunlight and convert it into direct current electricity. The solar array may use crystalline or thin film flexible solar cells mounted on the curved surface of a deployable structure, such as the power sphere. The power sphere offers a high collection area, low weight, and low stowage volume, while eliminating the need for a solar pointing and tracking mechanism. When solar cells are mounted on a curved surface the indivisible solar cells will each receive a different amount of incident light, and hence the indivisible solar cells will each be producing a different amount of current. In a spherical solar array of this curved arrangement, the solar cells are connected to the respective regulators for coupling respective solar cell output power on to the regulated bus. That solar cell generating the smallest amount of current will not limit the power output of any of the other cells coupled to the bus. Mismatches in current production are solved by the respective direct coupling connection of each indivisible solar cell to the regulated bus. This direct respective parallel connection is accomplished through the respective microelectronics DC-DC regulators. This direct connection eliminates the need for""series connections of these DC power sources and energy stores to efficiently produce sufficient voltage to supply the bus. If any one of the solar cells fails, the remaining cells still provide power to the bus through respective regulators to thereby gracefully degrade power production in the event of a solar cell failure. As such, this parallel configuration makes a spherical solar power sphere practicable for space missions.
The power distribution system enables the direct connection of an arbitrary number of individual indivisible DC power source devices and DC energy storage devices onto the bus. The power distribution system developed for the power sphere offers several advantages. Each power source device and energy storage device requires an electrical interface to the regulated bus providing necessary bus sensing and regulation. Direct connection of each individual DC power source device, such as solar cells, and DC energy storage devices, such as batteries, to a regulated bus is accomplished through the respective regulators providing respective regulated amounts of power. Regulation of the amount of current from the energy stores supplied to the bus at each instant of time is accomplished by respective boost converters within the respective regulators. Regulation of the amount of current from the power sources is accomplished using like boost converters in all of the respective regulators. In order to assure that the energy storage devices do not supply current to the bus when sufficient power is available from the power source devices, a bus voltage set point for the power sources is slightly higher than that for the energy storage devices. Hence, each of these DC power sources and energy storage devices have respective regulators that independently and respectively determine the solution to the energy balance equation for power coupling onto the bus. This energy balance equation solution is accomplished by sensing the bus voltage in comparison to a reference voltage and then adjusting the amount of current supplied to the bus through pulse width modulation. Each regulator must convert the voltage of the power source device or energy storage device to that of the regulated bus. Both of these tasks are preferably accomplished by use of similar pulse width modulated high frequency DC-DC regulators with associated sensing and control electronics.
For the energy storage devices, the bus interface electronics must be bidirectional, for energy storage at one instance and power delivery at another instance. In addition, the control circuitry of the electrical interface must have separate set points to determine when the bus requires current from the storage device to maintain energy balance, and when sufficient energy is available from the power sources on the bus for recharging the storage devices. Hence, each of the energy stores has a respective regulator for delivery of power onto the bus, and a respective charger for coupling power from the bus into the energy storage device. The interface electronic senses the amount of energy in storage device and terminates high rate charging when a full charge has been developed in the energy storage device. The interface electronics is also capable of providing a trickle charge rate to fully charge energy storage devices. The use of integrated circuit technology allows the packaging of the interface charging electronics into microchip integrated circuitry. This integration makes possible the deployment of a large number of individual energy storage devices with respective regulators and chargers each with respective control circuitry.
The power distribution system includes a plurality pulse width modulator regulators for obtaining regulated bus control through boost converters for the indivisible power sources and energy stores with switching step-up and step-down voltage regulation for load regulation to regulate the bus at a fixed voltage level. The pulse width modulated DC-DC regulation is being utilized for battery charge control. With the emergence of many new battery chemistries, it is becoming increasingly important for a charger to be able to handle multiple battery types. Various intelligent algorithms have been developed for accurately charging batteries so as to provide maximum storage capacity after each charge with increased overall battery cycle life. The chargers enable battery voltage sensing, current sensing, and temperature sensing multiplexed through an A/D converter, to a microcontroller allowing for a smart-charge control and monitoring system to operate the batteries at desired levels.
In general, all component requirements are met by standard commercial-off-the-shelf parts to implement the power sphere and power distribution system. The power distribution system architectural concept developed for the power sphere accommodates graceful degradation in the event of a failure and enables arbitrary orientation to the sun by connecting the indivisible devices in parallel to a regulated spacecraft power bus. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.